Tires

A tire is a covering mounted on the rim of a wheel to serve as a cushion and a gripping surface. Modern tires are made of fabric and rubber. Most are pneumatic, holding air under pressure, but some are solid. Formerly, all pneumatic tires had an inner tube to contain the air, but now most are tubeless and form an airtight seal with the rim of the wheel. Tires are used on all road vehicles, on aircraft landing gear, on tractors and other off-the-road equipment, on factory and warehouse mobile machinery, and on such diverse vehicles as shopping carts and baby carriages. Indoor tires are generally solid rubber and have smooth surfaces. Outdoor tires are generally pneumatic and have a pattern, called a tread, incised on the gripping surface, which is also called a tread.

History

In ancient times, wheel rims were protected by metal bands. The use of rubber did not become feasible until 1839, when Charles Goodyear discovered the process of vulcanization. In 1845, pneumatic tires were invented by Robert W. Thompson, a Scot, but they were not durable. In following decades, hundreds of kinds of solid tires of leather, rubber, and other materials were patented. However, the tire industry did not begin in earnest until pneumatic tires were refined and patented in 1888 by John Boyd Dunlop.

The first pneumatic tires in the United States were made by the Hartford Rubber Works in 1895. This company later became part of the United States Rubber Company. Other leading firms in the field are Goodyear, Firestone and Yokohama. These companies and a dozen other produce hundreds of millions tires a year for trucks, buses and automobiles for the US market alone.

Parts of a Tire

The main parts of the pneumatic tire are the tread, the body, and the beads. The tread is the part in contact with the road or other surface. It is a thick pad of rubber, with grooves cut into it to form cleats or ridges. It must provide traction both to move and halt the vehicle, and to prevent skidding and sliding while the vehicle is in motion. In tractor tires and snow tires the tread has deep grooves that enable the tire to move through soft earth or deep snow.

The body gives the tire its form and strength. It consists of layers of fabric permeated with rubber and with added synthetics. These layers are called plies, and the strength of a tire is sometimes indicated in terms of the number of plies in its body. Most automobile tires have two or four plies. All tires must be extremely strong to support the weight of vehicles. Tires on the landing gear of aircraft are under a particularly great strain. For example, the tires on the eight main wheels of a Boeing 707 jet must absorb the impact of a 247,000 pound (111,900 kg) airplane landing at a speed of 140 miles (225 km) per hour.

The beads of a tire are two strips that hold the tire to its wheel. They are located at the tire's inner edges and are made up of strands of wire surrounded with rubber and covered with fabric. In tubeless tires the beads form an airtight seal.

Tire Sizes

Pneumatic tires are made in a large variety of sizes. The size of a tire is usually indicated by an expression such as 7.50 x 14. The first term indicates the width, in inches, of the tire's tread, in this case 7 1/2 inches (19 cm). The second term indicates the diameter, in inches, of the wheel that the tire fits, in this case 14 inches (35 cm). A 7.50 x 14 tire would be used on a medium-size car. A large car might require a 9.00 x 15 tire, while a compact car or a sports model might use a 6.00 x 13. A golf cart and a wheelbarrow might have 4.00 x 8 tires, while huge earth-moving equipment may use 60 x 58 tires. Such tires are almost twice as tall as a man and weigh nearly 6,000 pounds (2,700 kg).

Tire Manufacture

Rubber, both natural and synthetic, is the basic ingredient in tire manufacture. Sulfur and other chemical activators are required for the process of vulcanization, which gives rubber its useful properties. Carbon black is added to improve resistance to abrasion. It consists of finely divided carbon particles, which give the tire its black color. Fabric and, in some tires, steel add to strength and resilience and are important in maintaining shape.

The first step in tire manufacture is the preparation of the crude rubber. It is broken down and softened by milling or mixing machines. Carbon black, sulfur, and other chemicals are added, in proportions that vary according to the part of the tire for which the batch of rubber is destined.

The principal parts of the pneumatic tire are made separately. The material for the body is worked into sheets on a rubber mill, which consists of two steel rollers between which the rubber is passed. The rubber is then pressed into and through sheets of fabric to make rubberized fabric. This material is cut into strips of various sizes, according to the types of tires to be manufactured.

The rubber for the tread is formed into the proper shape by extrusion, or forcing it through a die. The sidewall, the outer side of a tire, may be formed in one piece with the tread body or else it may be extruded separately. The beads are formed from metal strands coated with rubber and fabric.

The tire parts are joined together on a tire-building machine, which consists chiefly of a rotating collapsible drum. For a typical automobile tire, two plies are placed around the drum first, one on top of the other.

Then the beads are set into place and locked in by additional plies placed over them. Finally, the tread and sidewalls are put on top of the body plies to complete the tire, which now looks like a rubber barrel with no top or bottom. The forming drum is collapsed to permit its removal.

The next step is to vulcanize the rubber in the tire, bond the parts together, and mold the tire into its final shape. In one process the tire is first shaped on a hydraulic press and then placed in a heated mold for vulcanizing, bonding, and forming treads. In another process the mold shapes the tire and then heats it to vulcanize it and bond the parts together, impressing the tread pattern at the same time. After removal from the mold, tires are cooled gradually, while the process of vulcanization continues.

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